Communication receivers are used to receive desired carrier signals in narrow frequency bands. Such receivers typically include a static filter that filters out all but a predetermined band of desired frequencies. Automatic Frequency Control (AFC) systems are known, which improve sensitivity in radios having modest frequency stability performance. In AFC systems, the output of a receiver discriminator is low pass filtered at a very low frequency. The output of the filter is then driven to zero volts by adjusting the frequency of a local oscillator. An advantage of this system is that there is no ambiguity about which way to adjust the frequency of the local oscillator to compensate for the frequency offset and interference. In these systems, the low pass filtered discriminator output is proportional, in both magnitude and sign, to the local oscillator adjustment required.
AFC systems operate under the assumption that there is negligible low frequency content in the desired signal. The AFC systems also operate with the assumption that centering the signal maximizes the signal-to-noise ratio. These assumptions work well for 25 kHz and 30 kHz channel land-mobile radio systems where adjacent channel interference is negligible. In these systems, adjacent channel interference protection levels of 80 dB or more are common.
Increasingly, however, land mobile radio users are requiring spectrally efficient high-speed digital systems. In response, one standards setting body has standardized a 9.6 kbps, 12.5 kHz channel system and is developing a 12 kbps, two-slot TDMA 12.5 kHz channel system standard. Further, the Federal Communications Commission is splitting 25 kHz and 30 kHz channels into 12.5 kHz channels. As this narrow-banding occurs, it reduces the adjacent channel interference protection levels by 20 dB or more. Thus, in these 12.5 kHz channel systems, substantial adjacent channel interference is present. This interference is expected to degrade coverage in some frequency bands and geographical areas, as compared to what is currently available.
As a result of the increase in adjacent channel interference present when narrow-banding, the approach of minimizing frequency offset is not necessarily the most beneficial. In fact, in some interference cases it is better to adjust the receiver away from the desired signal. In these cases, the sensitivity degradation is minimized at a frequency in the opposite direction from the desired transmission.
In U.S. Pat. No. 5,465,410, Method and Apparatus for Automatic Frequency and Bandwidth Control, a quality metric was generated based on the output of each intermediate frequency (IF) filter in a multi-IF filter receiver and the output from the IF filter with the best quality metric was selected as the output from the receiver. This can improve simulcast performance. The patent suggested that decision error, which is the square of the symbol error, would be the preferred approach for determining the quality metric for a digital signal. Further, the patent suggested that the filter that minimized the decision error should be chosen. Unfortunately, it is impossible to effectively mitigate simulcast distortion as well as adjacent channel interference with this configuration, because it is necessary to react quickly to the changes in the level of symbol error because simulcast distortion is related to the phase relationship of the simulcast carriers. So, a new method of selecting the best IF in the presence of both adjacent channel interference and simulcast interference is needed.